Technique for insulation monitoring in vehicles

ABSTRACT

A device, in at least two measuring intervals, detects a respective operating state of at least one power converter of the motor vehicle, which is conductively connected to the traction energy store. The device further comprises a measuring device having a measuring terminal, which is conductively connected or connectable to least one DC voltage pole of an electric traction energy store of the motor vehicle, and a ground terminal which is conductively connected or connectable to a reference potential of the motor vehicle. The measuring device, in the at least two measuring intervals, respectively measures a conductance between the measuring terminal and the ground terminal. The device further comprises a calculation device, which determines the insulation resistances as a function of the at least two measured conductances and the at least two operating states detected.

FIELD OF THE DISCLOSURE

The present disclosure relates to a technique for determining insulationresistances. Specifically, a device for determining insulationresistances in a vehicle and a motor vehicle equipped with such a deviceare described.

BACKGROUND

Motor vehicles with an electric drive train are equipped with a DC powergrid, which distributes the electrical energy from a traction energystore (for example, a drive battery or a fuel cell) to electrical units,specifically to the electric drive unit of the drive train and auxiliaryunits of the motor vehicle. In general, the electrical units are notsupplied with the DC voltage from the DC power grid, but with analternating current, a multi-phase alternating current, a pulsed DCvoltage or a DC voltage with a rating which differs from that of the DCpower grid. Accordingly, power converters are arranged between the DCpower grid and the respective units. The function of the powerconverters is the conversion of direct current from the DC power gridinto the respective current profile, and vice versa, for example for therecovery of kinetic energy of the motor vehicle. Accordingly, coupled DCand AC power grids are present.

As a result of the high power demand of the electric drive system, theDC power grid and the AC power grids supplied by the latter arefrequently configured in the form of a high-voltage network (forexample, with voltages between 300 volts and 1000 volts) or IT (from theFrench “Isolé Terre” or English “insulated ground”) network. In order toensure electrical safety, particularly for vehicle passengers, duringoperation, the insulation resistance of the electric power grid inrelation to electrical ground is monitored. If the insulation resistancefalls as a result of insulation faults, a motor vehicle user can bealerted, for example, so that measures for the repair of the power gridcan be implemented promptly.

Conventionally, the insulation monitoring unit is electrically connectedto the DC power grid of the motor vehicle, for example within the drivebattery. In the prior art, insulation monitoring devices which employ apassive measuring method are widespread, as these devices are verycost-effective. By this arrangement, the DC voltage source which ispresent on the DC power grid is used to drive a measuring currentbetween the overall network and electrical ground. From the ensuingmeasuring current, the insulation resistance of the overall network inrelation to electrical ground can be determined by a known method.

However, the passive measuring method linked to the DC power grid isinaccurate and, in many cases, systematically incorrect, as the value ofthe insulation resistance of the AC power grid coupled via the powerconverters is not correctly determined. The measuring current dictatedby the insulation resistance of the AC power grid is reduced by theoperation of the power converters, and is customarily changed in atemporally varying manner. Thus, the insulation resistance of the ACpower grid measured using a conventional monitoring device from the DCpower grid is generally significantly higher than the actual value ofthe insulation resistance. Consequently, there is a risk than anin-service insulation fault on the AC power grid will not be detected.In summary, conventional devices employing the passive measuring methodon the DC power grid are not appropriate for the monitoring ofgalvanically-coupled AC power grids.

In comparison with passive measuring methods, active measuring methods,in which a time-coded voltage signal drives the measuring current, whichis measured by a lock-in amplifier, are cost-intensive and, as a resultof their complexity, more susceptible to errors.

Document DE 10 2010 054 413 A1 describes a method for the location of aninsulation fault in a system which comprises a DC section and an ACsection, incorporating an inverter. The inverter comprises powerswitches which, during a given measuring interval, are permanently open(freewheeling) and permanently closed (switched-through state),respectively. As such circuit states do not occur during the operationof the motor vehicle, and cannot be implemented without the restrictionof vehicle operation, this method cannot be deployed during vehicleoperation.

SUMMARY

The object is therefore the disclosure of a technique for determiningthe in-service insulation resistances of coupled DC and AC power grids.

This object is fulfilled by a device for determining insulationresistances in a motor vehicle, and a motor vehicle having thecharacteristics of the independent claims. Advantageous embodiments andapplications are the subject of the dependent claims, and are describedin greater detail hereinafter, with reference to the figures in someinstances.

According to a first aspect, a device for determining insulationresistances in a motor vehicle comprises a detection device which isconfigured, in at least two measuring intervals, to detect a respectiveoperating state of at least one power converter of the motor vehicle,which is conductively connected to a traction energy store; a measuringdevice with a measuring terminal, which is conductively connected orconnectable to at least one DC voltage pole of an electric tractionenergy store of the motor vehicle, and a ground terminal, which isconductively connected or connectable to a reference potential of themotor vehicle, wherein the measuring device is configured, in the atleast two measuring intervals, to respectively measure a conductancebetween the measuring terminal and the ground terminal; and acalculation device, which is configured to determine the insulationresistances as a function of the at least two measured conductances andthe at least two operating states detected.

Conductance may be determined in an equivalent manner by thedetermination of resistance. Conductance and resistance may be mutuallyreciprocal.

The traction energy store may comprise a plurality of cell modules forthe storage of electrical energy. The traction energy store may furthercomprise an intermediate circuit and/or an intermediate circuit voltageconverter. The intermediate circuit voltage converter, on the inputside, may be connected to the cell modules. The converter may beconnected on the output side to the intermediate circuit. The DC voltagepole may be an output-side pole of the intermediate circuit.

Measurement of conductance may be executed by the measurement of overallresistance. The measuring device, in the respective measuring interval,may measure current and voltage between the measuring terminal and theground terminal, wherein the conductance or overall resistance arecalculated (for example by the measuring device or the calculationdevice) from the measured current and the measured voltage (for exampleas a ratio).

The operating state may comprise a circuit state and/or a duty factor ofat least one switching element of the power converter. The operatingstate may be represented by a coupling factor (for example, a realnumber between 0 and 1). The insulation resistance determined may (forexample in the case of conductance determined by the measurement) be astrictly monotonically declining function of the duty factor or thecoupling factor. For example, the inverse value of the insulationresistance may be linear, both in the measured conductance and in theduty factor or coupling factor. Alternatively, the insulation resistancemay be linear, both in relation to the measured resistance (i.e. theinverse value of the conductance) and to the inverse duty factor orcoupling factor.

The duty factor detected may be less than 100% and/or greater than 0%,for example greater than 10%. The coupling factor corresponding to theduty factor detected (for example, in the case of a multiplicativeassociation of the coupling factors with the respective conductances)may be less than 1 and greater than 0, for example greater than 0.1. Thecoupling factor corresponding to the duty factor detected (for example,in the case of a multiplicative association of the coupling factors withthe respective resistances) may be greater than 1 and, optionally, lessthan 10. The duty factor detected may be greater than a minimum value(for example 5%, 10% or 20%).

Each of the operating states detected may be temporally constant duringthe respective measuring interval. The combination (or assembly) of theoperating states detected in each measuring interval may be different ineach measuring interval. Alternatively or additionally, the detectiondevice determines a time-averaged state for a temporally varyingoperating state in the respective measuring interval. The time-averagedstate is considered by the calculation device as a basis for determiningthe insulation resistances.

The at least one power converter may comprise an inverter and/or a DCconverter. Each of the power converters may incorporate field effecttransistors as switching elements.

The at least one power converter may respectively supply a unit of themotor vehicle, or feedback electrical energy from the latter to the DCpower grid. The unit may comprise an electrical machine (drive unitand/or generator).

The reference potential may be constituted by an electrically conductivebodywork of the motor vehicle. Bodywork parts which are electricallyseparated by composite fibre materials may be electrically connected bygrounding cables.

The detection device may be configured to respectively detect theoperating states of a plurality of power converters in a plurality ofmeasuring intervals. To this end, the measuring device may be configuredfor the measurement of conductance in each of the measuring intervals.To this end, the calculation device may be configured for thecalculation, for each of the power converters (or each switchingelement), of an insulation resistance on the basis of the measuredconductances and the operating states detected.

One of the power converters (for example, an inverter) may beconductively connected to an electrical machine for the propulsion ofthe motor vehicle. Alternatively or additionally, one of the powerconverters (for example, a DC voltage converter) may be conductivelyconnected to an electric drive mechanism for the power-assisted steeringsystem of the motor vehicle. Alternatively or additionally, one of thepower converters (for example, a DC voltage converter or converterdevice) may be conductively connected to a compressor or a pump in themotor vehicle, for example the compressor of an air-conditioninginstallation, the compressor for the compression of air, or the oil pumpin a hydraulic system of the motor vehicle.

The function may incorporate parameters or coupling factors for theweighting of a dependence of the insulation resistance on the operatingstate detected. The calculation device may incorporate a memory, inwhich motor vehicle-specific parameter values are saved. The functionmay incorporate parameters or coupling factors for the weighting of alinear dependence and/or a non-linear dependence of the insulationresistance on the operating state detected.

The detection device may define the measuring intervals in accordancewith a driving state of the motor vehicle. The detection device maydefine at least one measuring interval, during which at least one or allof the operating states are temporally constant.

According to a further aspect, a motor vehicle is provided, having adevice according to the above-mentioned aspect. The motor vehicle maycomprise an electric traction energy store, at least one power converterwhich is conductively connected to the traction energy store, and thedevice for determining insulation resistances in the motor vehicle.

The motor vehicle may be a utility vehicle, for example a heavy goodsvehicle, or a tractor vehicle (for the conveyance of goods) and/or a bus(for the conveyance of persons), or a passenger motor vehicle.

BRIEF DESCRIPTION OF THE DRAWINGS

Features described above can be implemented in any combination. Furtherfeatures and advantages of the disclosure are described hereinafter withreference to the attached drawings. In the figures:

FIG. 1 shows a schematic representation of the electric power grid of amotor vehicle for determining insulation resistances according toembodiments of the present disclosure;

FIG. 2 shows a schematic block circuit diagram of the device fordetermining insulation resistances in a motor vehicle according toembodiments of the present disclosure; and

FIG. 3 shows a schematic circuit diagram for calculating the insulationresistances, which may be implemented in one of the embodiments of thedevice.

DETAILED DESCRIPTION

FIG. 1 shows a schematic representation of a motor vehicle 100, in whichthe present technique may be implemented. The motor vehicle 100comprises an energy store-side region 102 and a vehicle-side region 104.The energy store-side region 102 and the vehicle-side region 104 areconnected by a DC power grid 106 to which, in the vehicle-side region104, at least one AC power grid 108 is coupled. Typically, at least oneAC power grid 108 is supplied from the DC power grid 106. The coupledpower grids 106 and 108 are also described as an overall network, andmay be configured as an IT network.

The motor vehicle 100 comprises a traction energy store 110 for thestorage of electrical energy. Said energy is delivered via DC voltagepoles 112 to the DC power grid 106 (and, where applicable, in the caseof energy recovery, fed to the traction energy store 110). The tractionenergy store 110 comprises a plurality of electrochemical cells, whichare interconnected in cell modules 114, and are switchably connected tothe DC voltage poles 112 via a contactor 116.

The motor vehicle 100 further comprises, in the vehicle-side region 104,at least one power converter 120 which is connected to the tractionenergy store 110 in an electrically conductive manner, and whichrespectively supplies at least one unit 130 of the motor vehicle 100 inthe respective AC power grid 108. To this end, the power converter 120comprises switching elements 122 which are respectively connected on theinput side to the traction energy store 110, and on the output side tothe respective unit 130 of the motor vehicle 100. Typically, anintermediate circuit 118 is connected between the cell modules 114, asthe primary voltage source, and the input side of the switching elements122, which intermediate circuit may be an element of the traction energystore 110, a stand-alone element of the DC power grid 106 or (asrepresented in FIG. 1) an element of the power converter 120.

The unit 130 represented in FIG. 1 comprises an electrical propulsionmachine (also designated as an electric traction machine, or ETM) of themotor vehicle 100, which machine is configured as a multi-phase ACmachine. The respective phases 132 are controlled by two switchingelements 122, which are respectively configured as half-bridges. Themulti-phase AC power grid 108 represented in FIG. 1 is shown as anexample. The technique may be employed with any current profile, forexample single-phase AC and/or a DC voltage controlled by pulse widthmodulation (or PWM). Specifically, the motor vehicle 100, from the DCpower grid 106, may supply a plurality of AC power grids 108 which, forexample, each deliver different current profiles for different units130.

According to one operating state of the power converter 120, theswitching elements 122 thereof assume a (generally dynamic) circuitstate. The function of the switching elements 122 is in this case theconversion of the direct current tapped from the DC power grid 106 intoa correspondingly constituted alternating current on the AC power grid108 and, where applicable, vice versa, for example for energy recovery.The switching elements 122 may comprise switching semiconductor elementswhich, in the operation of the motor vehicle 100, in accordance with a(generally time-dependent) circuit state, are actuated at high switchingfrequencies, for example at least 1 kHz.

The circuit state, for example, is determined by a duty factor (alsodescribed as a pulse duty ratio). The duty factor of the switchingelements 122 constitutes one example of a parameter for the operatingstate of the power converter 120. The duty factor is the ratio of thepulse duration with the switching element 122 closed to the periodduration of the dynamic circuit state (i.e. of the reciprocal switchingfrequencies).

For the determination of insulation resistances of the power grids 106and 108 in relation to an electrical ground 140 of the motor vehicle100, for example the reference potential defined by a metal vehiclechassis, the motor vehicle 100 comprises a device 150 for determininginsulation resistances in the motor vehicle 100. The device 150 may alsobe designated as an “Insulation Monitoring Device” (or IMD).

The device 150 represented in FIG. 1 comprises at least one measuringterminal 152, which is conductively connected to at least one DC voltagepole 112 or 113 of an electric traction energy store 110. The device 150further comprises a ground terminal 153, which is conductively connectedto a reference potential 140 of the motor vehicle 100.

The device 150 can, for example, as schematically represented in FIG. 1,be arranged within the traction energy store 110. Alternatively, thedevice 150 may be arranged as a stand-alone unit in the DC power grid106. Moreover, the device may be partially integrated in the tractionenergy store 110. Advantageously, the measuring terminal 152 isconnected to a DC voltage pole 113 of the cell module 114, independentlyof the state of the contactor 116, for example for the determination ofinsulation resistances with the contactor 116 open.

For the determination of insulation resistances, the device 150 detectsthe operating state of the power converter 120, i.e. a circuit statedefined by a temporal switching characteristic of the switching elements122, for example an operating state of at least one switching element122, which operating state is defined by a duty factor. In a pluralityof temporally separate measuring intervals, the device 150 respectivelymeasures the conductance between the measuring terminal 152 and theground terminal 153. The operating state defines a circuit state or dutyfactor that is stationary for the respective measuring interval anddynamic in general (for example between the measuring intervals). Forexample, the at least one switching element 122 executes a plurality ofopening and closing cycles (for example, opens and closes more than 100or 1000 times) within a measuring interval, according to the circuitstate or duty factor.

The device 150 employs the operating state of the power converter 120,i.e. the circuit state or duty factor of the switching elements 122which connect the DC power grid 106 to the AC power grid 108, for theaccurate determination of the value of the insulation resistance,including that of AC power grids 108. The circuit state or duty factorof the switching elements 122 may be actively influenced by a detectiondevice in the device 150 for the determination of insulation resistanceand/or may be provided to the latter. For example, the detection devicecomprises an interface with the power converter 120 for the detection orscanning of the operating state of the power converter 120. For example,status settings or status signals are exchanged via the interface, e.g.a closed position or open position of one of the switching elements 122,or a duty factor of a semiconductor converter as the power converter120.

FIG. 2 shows a schematic block circuit diagram of the device 150 for thedetermination of insulation resistances in a motor vehicle 100 accordingto another embodiment of the present disclosure. Features which areinterchangeable or consistent with those shown in FIG. 1 are identifiedby the same reference numbers.

The device 150 comprises a detection device 156 which is designed, in atleast two measuring intervals, to detect a respective operating state ofat least one power converter of the motor vehicle 100, which isconductively connected to the traction energy store 110, or theswitching elements 122 of said power converter.

The device 150 further comprises a measuring device 154 having ameasuring terminal 152, which is conductively connected to at least oneDC voltage pole 112 of an electric traction energy store 110 of themotor vehicle 100, and a ground terminal 153 which is conductivelyconnected to a reference potential 140 of the motor vehicle 100. Themeasuring device 154 is designed, in the at least two measuringintervals, to respectively measure at least one conductance between themeasuring terminal 152 and the ground terminal 153.

To this end, the measuring device 154 may be configured in a spatiallyseparate arrangement from other facilities in the device 150, forexample within the traction energy store 110. The detection device 156may be configured as a superordinate facility, which coordinates thepassive detection of the operating states of the power converter 120(for example, the circuit states or duty factors of the switchingelements 122) with the measuring device 154 for the measurement ofconductance.

The device 150 further comprises a calculation device 158, which isdesigned to determine the insulation resistances as a function of the atleast two measured conductances and the at least two operating statesdetected. The calculation device 158 may, as represented in FIG. 2, bean element of the detection device 156.

FIG. 3 shows a circuit diagram of the motor vehicle 100, with respect tothe insulation resistance 162 (designated as R_(Iso,DC)) of a DC powergrid 106 and the insulation resistances (designated as R_(Iso,AC1) andR_(Iso,AC2), respectively) of two AC power grids 108. The circuitdiagram, or the corresponding calculation by the calculation device 158may be implemented in each embodiment of the device 150. In eachmeasuring interval, the conductance 160 is measured between themeasuring terminal 152 and the ground terminal 153 (or, as anequivalent, the inverse value thereof, as a combined insulationresistance).

The respective AC power grids 108 are parallel-connected to the DC powergrid 106 via the switching elements 122 of the associated powerconverter 120. The coupling of the AC power grids 108 via the respectivepower converters 120 is incorporated in the determination of theinsulation resistances by means of factors K₁ or K₂ which are determinedfrom the operating state 124 of the respective power converter 120 orswitching element 122. For example, the factor K_(i) corresponds to theduty factor of the respective power converter 120 or switching element122.

The power converters 120 may be configured by means of a powerelectronic device (PE). Specifically, the power converters 120 maycomprise an AC/DC converter and/or a DC/AC converter. The respectiveoperating states 124 detected in the at least two measuring intervalsmay respectively comprise an active operating state (index “A”) orpassive operating state (index “P”). The active operating state maycomprise, for example, a duty factor of 50%, i.e. the controlled unit isin service. The passive operating state may comprise, for example, aduty factor of 0% or less than 5%, i.e. the controlled unit is not inservice. The terms “active” and “passive” are designations for twodifferent operating states.

If the switching elements 122 are completely closed for the duration ofthe measuring interval (e.g. in the event of an active short-circuit ofthe AC phases of all the converters 120, wherein the three AC phasesthereof are respectively connected to one DC voltage pole, i.e. only theuppermost or only the lowermost three switching elements 122 are closed,as both DC voltage poles would otherwise be short-circuited likewise), asingle measurement is sufficient for the accurate determination orevaluation of the value of the insulation resistance of the overallnetwork. Thus, on the basis of parallel connection, it may be determinedor evaluated that the insulation resistances of the individual phases(i.e. the individual values for the subnetworks) are all equal to orgreater than the combined measured value for the insulation resistance.Compliance with a limiting value may thus be ensured, with no safetymargin. It may be required that the resistance of the converter, in theevent of an active short circuit, should be negligible, i.e. anapproximation to a coupling factor equal to 1. The closing of theswitching elements 122 is in this case executed in temporal coordinationwith the measurement.

The calculation device 158 determines the insulation resistances 162,according to the parallel connection of the electric circuits 106 and108 in the respective operating state 124 and the measured conductance160. The effect of the operating state 124 of the power converter 120,or the switching elements 122 thereof, (e.g. the duty factor of aconverter 120) may be determined analytically or experimentallybeforehand, and saved in the calculation device 158, e.g. by means ofthe factors K_(i), depending on the duty factor, in the simplest form,or in the form of linear or non-linear functions (for example, in aseries expansion). These functions may be determined, for example in avehicle-specific or type-specific manner (for a serial model), prior tothe manufacture of the motor vehicle 100 (for example, using a testset-up, based upon known insulation resistances in the AC power grid108, or using a prototype of the motor vehicle 100).

For an overall network comprising a DC power grid 106 and an AC powergrid 108, at least two measurements of conductance 160 must be executed,i.e. two detected measuring intervals, with at least two differentoperating states 124 (for example, circuit states or duty factors of theswitching elements). In general, the N insulation resistances of N ACpower grids 108 and the insulation resistance of one DC power grid 106may be determined on the basis of N+1 measurements of the conductance160 in one measuring interval respectively, with a corresponding numberN+1 of different operating states. If more than N+1 measurements areavailable, the calculation device 158 may execute a linear regression.

The passive method executed by the detection device 156 comprises thedetection or scanning of different operating states 124 (for example,circuit states or duty factors) of the power converter 120 or theswitching elements 122 thereof. Accordingly, operating requirements forthe converter 120 (or operating requirements for the switching elements122 thereof) do not impair user functions (e.g. driving, cooling,power-assisted steering).

For example, the operating states 124 change in response to operatingrequirements (e.g. a power demand for the AC machine), specifically bymeans of the time-variable actuation of the duty factor of the powerconverter 120 for the propulsion system (according to the torquerequired by the driver), of a power converter 120 for power-assistedsteering (according to the steering angle), or of a power converter 120for the air-conditioning compressor (according to cooling requirements).The detection device 156 detects the operating requirement and, on thisbasis, determines the measuring interval and the operating states 124.For example, the operating requirement may incorporate a predefinedprofile of the operating states 124. The detection device may determinethe measuring interval in a phase of temporally constant operatingstates. Alternatively, or in combination with partially temporallyconstant operating states 124, time-variable operating states 124 aredetermined over the measuring interval. This facilitates the accuratedetermination of the value of the conductance 160, or of thecorresponding combined insulation resistance.

Alternatively or additionally, the duration of the measuring interval(e.g. 500 ms to 30 s) is dependent upon the variability of the operatingstate 124 detected. A minimum duration of the measuring intervals (i.e.of measuring times) may be established on the basis of the dischargecapacities of the IT-network and/or by filtering (executed by themeasuring device 154).

If a minimum of two measured values for the conductance 160 (forexample, the combined insulation resistance) and, respectively, theoperating state 124 (optionally arranged) (for example, the associatedfunctional dependence of the insulation resistance 162 in the respectiveoperating state 124, or values for the factors K_(i), according to theoperating state 124) are known, the calculation device 158 determinesthe unknown variables 162 of the insulation resistance R_(Iso,DC) in theDC power grid 106 and the respective insulation resistance R_(Iso,ACi)in the i-th AC power grid 108, for example by means of regression. Inprinciple, the calculation device 158 permits the determination of allinsulation resistances 162, if the number of measurements (i.e. ofmeasuring intervals) is greater than or equal to the number ofsubnetworks 106 and 108.

For example, the calculation device 158 calculates the insulationresistances 162 on the basis of the measured conductances 160 (L_(i) orthe combined insulation resistances R_(Iso,i)=1/L_(i)) and therespective coupling factors K_(i) for operating states 124, as follows.

On the basis of, for example, N+1=3 measuring intervals, the data sets(R_(Iso,1), K_(1P), K_(2P)), (R_(Iso,2), K_(1A), K_(2P)), (R_(Iso,3),K_(1A), K_(2A)) are known. The insulation resistances 162, i.e.R_(Iso,DC), R_(Iso,AC1), R_(Iso,AC2), for the respective power grids 106and 108 are calculated. More specifically, the calculation device 158determines the insulation resistances 162 as a function of the datasets, based upon functional dependencies in the respective measuringinterval:Measuring interval 1: R_(Iso,1)=R_(Iso,DC)∥K_(1P) R_(Iso,AC1)∥K_(2P)R_(Iso,AC2)∥ . . .Measuring interval 2: R_(Iso,2)=R_(Iso,DC)∥K_(1A) R_(Iso,AC1)∥K_(2P)R_(Iso,AC2)∥ . . .Measuring interval 3: R_(Iso,3)=R_(Iso,DC)∥K_(1A) R_(Iso,AC1)∥K_(2A)R_(Iso,AC2)∥ . . .

Herein, “∥” represents the parallel electrical connection ofresistances, i.e. the sum of the inverse values of the resistances. Ifthe data sets for N+1 measuring intervals are available to thecalculation device 158, all insulation resistances 162 may be calculated(for detected operating states that are disjunct), for example by theinversion of a matrix determined by the coupling factors. If data setsare available for more than N+1 measuring intervals, the insulationresistances 162 are determined by regression.

For example, the insulation resistances are calculated as follows:Measuring interval 1: R _(Iso,1)=1/(1/R _(Iso,DC)+1/(K _(1P) ·R_(Iso,AC1))+1/(K _(2P) ·R _(Iso,AC2))+ . . . )Measuring interval 2: R _(Iso,2)=1/(1/R _(Iso,DC)+1/(K _(1A) ·R_(Iso,AC1))+1/(K _(2P) ·R _(Iso,AC2))+ . . . )Measuring interval 3: R _(Iso,2)=1/(1/R _(Iso,DC)+1/(K _(1A) ·R_(Iso,AC1))+1/(K _(2A) ·R _(Iso,AC2))+ . . . )

Alternatively, reciprocal coupling factors (for example, in acalculation involving the conductances) may be applied:Measuring interval 1: G _(Iso,1) =G _(Iso,DC) +K _(1P) ·G _(Iso,AC1) +K_(2P) ·G _(Iso,AC2)+ . . .Measuring interval 2: G _(Iso,2) =G _(Iso,DC) +K _(1A) ·G _(Iso,AC1))+K_(2P) ·G _(Iso,AC2)+ . . .Measuring interval 3: G _(Iso,2) =G _(Iso,DC) +K _(1A) ·G _(Iso,AC1) +K_(2A) ·G _(Iso,AC2)+ . . .

The technique may be scaled to more than one AC power grid 108 (forexample, a plurality of AC machines, e.g. propulsion systems and/orauxiliary units), wherein the number of measuring intervals required (atleast N+1), for respectively different operating states 124, isincreased in order to permit the determination by the device 150 of allthe insulation resistances 162 in the overall network.

Whereas a distinction has been drawn heretofore between the respectiveassociation of power inverters 120 with an AC power grid 108 and therespective association of the switching elements 122 thereof with onephase of the AC power grid 108, the device 150 may also be employed forthe determination of the insulation resistances of the individualphases. To this end, the approximationload impedance<<insulation resistanceis applied. In this case, the terms “power inverter” 120 and “switchingelements” 122 may be interchangeable.

As will be evident to a person skilled in the art from theabove-mentioned embodiments, the technique permits the cost-effectivedetermination of insulation resistances in coupled electric power gridsof a motor vehicle during operation.

The technique permits the implementation of a cost-effective passivemeasuring method for the accurate determination of the value of theinsulation resistance in coupled AC power grids.

Conversely, according to the prior art, a significantly morecost-intensive, active measuring method is required for this purpose todate, involving a test voltage which is modulated according to thenetwork voltage.

The technique may determine individual insulation resistance values ineach subnetwork (specifically in each DC and AC power grid). Thus, bythe application of the technique, the location of an insulation faultmay also be advantageously determined. This facilitates faultidentification and fault clearance.

The targeted, passive detection of respectively constant operatingstates during the measuring intervals (for example, circuit states orduty factors) permits accurate measurements of the values and/orimproves the measuring accuracy of the accurate measurements of thevalue of the insulation resistance.

The operation of the DC and AC power grids (e.g. for the utilityfunction of driving of an electric vehicle) may remain unaffected by themethod, specifically wherein the measuring intervals occur before and/orafter the operating phase.

Although the disclosure has been described with reference toembodiments, it will be evident to a person skilled in the art thatvarious modifications may be undertaken, and equivalents may be appliedby way of substitution. Moreover, many modifications may be undertaken,in order to adapt a specific situation or a specific vehicle model tothe teaching of the disclosure. Consequently, the disclosure is notlimited to the embodiments disclosed, but encompasses all embodimentswhich fall within the scope of the attached patent claims.

LIST OF REFERENCE NUMBERS

100 Motor vehicle

102 Store-side region

104 Vehicle-side region

106 DC power grid

108 AC power grid

110 Traction energy store

112, 113 DC voltage pole

114 Cell modules

116 Contactor

118 Intermediate circuit

120 Power converter

122 Power converter switching element

124 Operating state of power converter

130 Unit

132 Phases of unit

140 Reference potential of motor vehicle

150 Device for determining insulation resistances

152 Measuring terminal

153 Ground terminal

154 Measuring device

156 Detection device

158 Calculation device

160 Conductance

162 Insulation resistance

We claim:
 1. A device for determining insulation resistances in a motorvehicle, comprising: a detection device which is configured, in at leasttwo measuring intervals, to detect a respective operating state of atleast one power converter of the motor vehicle, which is conductivelyconnected to a traction energy store; a measuring device with ameasuring terminal, which is conductively connected or connectable to atleast one DC voltage pole of an electric traction energy store of themotor vehicle, and a ground terminal, which is conductively connected orconnectable to a reference potential of the motor vehicle, wherein themeasuring device is configured, in the at least two measuring intervals,to respectively measure a conductance between the measuring terminal anda ground terminal; and a calculation device, which is configured todetermine the insulation resistances as a function of at least twomeasured conductances and at least two operating states detected.
 2. Thedevice according to claim 1, wherein each of the at least two operatingstates comprises a circuit state and/or a duty factor of at least oneswitching element of the at least one power converter.
 3. The deviceaccording to claim 2, wherein the insulation resistances determined area strictly monotonically declining function of the duty factor.
 4. Thedevice according to claim 2, wherein the duty factor detected is lessthan 100% and/or greater than 10%.
 5. The device according to claim 1,wherein the detection device is further configured to determine anaverage operating state for each of the at least two operating statesdetected in the respective measuring interval.
 6. The device accordingto claim 1, wherein the at least one power converter comprises aninverter and/or a DC converter.
 7. The device according to claim 1,wherein the at least one power converter respectively supplies a unit ofthe motor vehicle.
 8. The device according to claim 1, wherein thereference potential is constituted by an electrically conductivebodywork of the motor vehicle.
 9. The device according to claim 1,wherein the detection device is configured to respectively detect the atleast two operating states of a plurality of power converters in aplurality of measuring intervals, the measuring device is configured tomeasure conductance in each of the plurality of measuring intervals, andthe calculation device is configured for the calculation, for each ofthe plurality of power converters, of an insulation resistance on thebasis of the at least two measured conductances and the at least twooperating states detected.
 10. The device according to claim 1, whereinthe function incorporates parameters for the weighting of a dependenceof the insulation resistances on the operating state detected, andwherein the calculation device incorporates a memory, in which motorvehicle-specific parameter values are saved.
 11. The device according toclaim 1, wherein the function incorporates parameters for the weightingof a linear dependence and/or a non-linear dependence of the insulationresistances on the operating state detected.
 12. The device according toclaim 1, wherein the detection device defines the at least two measuringintervals in accordance with a driving state of the motor vehicle. 13.The device according to claim 12, wherein the detection device definesat least one measuring interval, during which at least one of the atleast two operating states is temporally constant.
 14. A motor vehicle,in particular a utility vehicle, comprising: an electric traction energystore; at least one power converter which is conductively connected tothe electric traction energy store; and a device for determininginsulation resistances in the motor vehicle comprising: a detectiondevice which is configured, in at least two measuring intervals, todetect a respective operating state of at least one power converter ofthe motor vehicle, which is conductively connected to the electrictraction energy store; a measuring device with a measuring terminal,which is conductively connected or connectable to at least one DCvoltage pole of an electric traction energy store of the motor vehicle,and a ground terminal, which is conductively connected or connectable toa reference potential of the motor vehicle, wherein the measuring deviceis configured, in the at least two measuring intervals, to respectivelymeasure a conductance between the measuring terminal and a groundterminal; and a calculation device, which is configured to determine theinsulation resistances as a function of at least two measuredconductances and at least two operating states detected.